The present invention relates to an electrode for the transfer of an electrical current traversing a patient""s skin, and more particularly, to such an electrode of the type which comprises a) an electricity conducting layer for the supply or the collection of said electrical current traversing the skin, and b) a consumable electrochemical interface layer.
Electrodes of this type are used in various medical applications, such as the recording of electrical signals originating from the human or animal body (electrocardiogram, electroencephalogram, etc. . . . ), the transdermal administration of a drug by ionophoresis or the electrocicatrization of wounds or injuries.
The devices designed to use these applications normally comprise two electrodes, one functioning as anode and the other as cathode, both placed in contact with the skin, where the current applied, or to be recorded, passes between these two electrodes.
Each electrode is formed in the standard way of a metal conducting layer, for example, made of silver, associated with (or glued to), an ionic conduction layer consisting of a gel in contact with the patient""s skin, where this gel is either charged with an electrolyte ensuring a good electrical contact between the skin and the silver layer (in the case of the recording of electrical signals originating from the human or animal body, for example) or it is charged with an active substance in ionic form (in the case of the transdermal administration of drugs by ionophoresis).
In the case of electrocicatrization of wounds or injuries, said ionic conduction layer associated with the conducting layer is a hydrophilic layer which in this case is in contact with the wound to be treated.
This hydrophilic layer is either a layer which was either initially rendered conducting, that is before its application to the wound, or immediately after the application, by a charge of water and mineral salts, or it is a dry hydrophilic layer which is nonconducting during its application to the skin, and then becomes conducting due to the migration of the exudates of the wound into the thickness of the said dry hydrophilic layer.
In this case, the dry hydrophilic layer which constitutes the ionic conducting layer can be prepared from any dry hydrophilic absorbing material of composition used in the preparation of dressings intended for the treatment of exudating wounds.
It can also be in the form of absorbing foams, in particular hydrophilic foams of polyurethane used in so called hydrocellular dressings, in the form of fibers based on absorbing materials such as, for example, alginate fibers, such as sodium or calcium alginates, or fibers of cellulose derivatives, in the form of compresses made of nonwoven materials, in the form of lyophilized gels and in the form of hydrocolloid compositions such as gels or hydrocolloid compositions, such as those used in dressings of the so called hydrocolloid type.
In all cases, the electrical conduction passes from electronic conduction (in the layer of silver) to ionic conduction (in the gel). The transfer of charge that allows this exchange is controlled by the electrochemical properties of the electrode/skin interface.
To ensure the stability of these electrochemical properties, it has been suggested to form on the above mentioned layer of silver an electrochemical interface layer which is such that it facilitates the passage from ionic conduction to electrochemical conduction by maintaining, at the interface in question, a weak and stable electrochemical potential, ensuring a high-yield charge transfer.
This interface layer, also called xe2x80x9csacrificialxe2x80x9d layer, conventionally consists of silver or a mixture of Ag/AgCl, in the case of an anode or a cathode, respectively, deposited on, or mixed with, the current supplying layer. The electrochemical interface specific for these sacrificial layers is then provided by the reversible redox reaction:
Ag+Clxe2x88x92⇄AgCl+exe2x88x92
which transforms, by oxidation, the silver of the anode into silver chloride and, by reduction, the silver chloride of the cathode into silver. The potential of the reaction is relatively weak (approximately 100-200 mV) compared to the potential of the standard hydrogen electrode. The difference in potential between the anode and the cathode is weak and of the same order of magnitude. In addition, because the standard potential of the Ag/AgCl pair is located in the window of electrochemical stability of the water, this reaction prevents any phenomenon of electrolysis of the water, and the accompanying variations in pH and formation of gas, and thus ensures the electrochemical stability of the interface and the safety of the patient with respect to one of the hazards connected with the drifting of the electrochemical interface, and this for as long as said reaction can take place, that is as long as silver chloride is present at the cathode and/or as long as silver and the Clxe2x88x92 ion are available at the anode.
Although such interface layers thus meet the requirement as far as the electrochemical stability of the electrode/skin interface is concerned, it was observed that, when the treatment requires the passage of strong currents for long periods of time, as is the case in the ionophoretic devices for transdermal administration of drugs, electrodes equipped with such interface layers still were unsatisfactory with regard to their resistivity to physical-degradation, the uniformity of the density of the current on the surface of the electrode, and the xe2x80x9celectrochemical capacityxe2x80x9d of the latter, that is its ability to deliver a current with a given intensity (on the order of 1 xcexcA to 1 mA, more specifically on the order of 0.2 mA per cm2 of electrode surface in the case of application in ionophoresis) for a given time interval (of several hours or more) as is required to ensure precise control of a transdermal administration of a drug by ionophoresis.
Indeed, one observes, in this application, some dislocation of the electrode, notably due to a deficiency in the adherence of the layer of Ag/AgCl to the underlying layer of silver. One also observes the absence of uniform density of the current traversing adjacent areas of the electrode, where this density tends to increase on the edges of the electrode. This deficiency of uniformity creates xe2x80x9chot spotsxe2x80x9d on these edges which irritate the patient""s skin. In addition, it causes a more rapid erosion of the electrochemical interface layer at places perpendicular to the hot spots of the electrode, whose life span is then decreased. This decrease results in a decrease of the electrochemical capacity of the electrode.
The present invention precisely has the purpose of providing an electrode of the above described type, which is improved so as to present a superior mechanical stability and electrochemical capacity, compared to those of the electrodes of the prior art, that is a life span which is compatible with the duration of the therapeutic treatments which use such electrodes, such as transdermal administration of drugs by ionophoresis, or electrocicatrization.
These purposes of the invention have been achieved, as well as others which will become apparent in a reading of the following description, with an electrode for the transfer of an electrical current traversing a patient""s skin, of the type which comprises a) an electricity conducting layer for the supply or the collection of said electrical current traversing the skin, and b) a consumable electrochemical interface layer, this electrode being remarkable in that it comprises, between said conducting layer and said electrochemical interface layer, c) an intermediate layer made of a chemically inert and electrically resistant material.
As will be shown in detail below, the presence of this intermediate layer simultaneously improves the mechanical resistivity of the electrode and its electrochemical capacity.
According to a preferred embodiment variant of the invention, the intermediate layer comprises carbon, for example, in the form of fine particles dispersed in a polymer binder.
According to another preferred embodiment variant of the invention, the electrode is formed on a substrate made of an electrochemically insulating material to which the electricity conducting layer is glued.
According to yet another preferred embodiment variant of the invention, the electrode in addition comprises an ionic conduction layer in contact with a patient""s wound or skin.
According to an optional characteristic of the electrode according to the invention, the latter in addition comprises a layer made of an electrically insulating material, which covers the edges of the stack of layers formed by the conducting layer, the intermediate layer and the electrochemical interface layer.
According to yet another optional characteristic of the electrode according to the invention, the conducting layer is in the form of a grid.